Cuts to two fiber lines caused a widespread system failure at cable giant
Comcast Corp.
CMCSA -0.19%
on Friday that knocked out cable, internet and phone services around the country.

It was unclear how many customers were affected as the system failure, which appeared contained to Comcast’s network, also disrupted connectivity services such as
Netflix Inc.
and
Okta Inc.
as other internet service providers routed internet traffic through Comcast’s network, according to network-monitoring firm ThousandEyes.

Philadelphia-based Comcast, one of the dominant telecom companies in the U.S. with more than 29 million business and residential customers, said the lines damaged are owned by
CenturyLink Inc.
and
Zayo Group Holdings Inc.

A spokeswoman for CenturyLink issued a statement saying CenturyLink’s network was working normally, though the company had “experienced two isolated fiber cuts in North Carolina affecting some customers that in and of itself did not cause the issues experienced by other providers.” The spokeswoman didn’t comment further.

A spokesman for Zayo said the company experienced a fiber cut in the New York area but all services had been since restored.

Fiber networks, which make up the backbone of the internet, transmit vast amounts of internet traffic, processing everything from online purchases to 911 calls.

Down Detector and Outage.Report, two websites that monitor the running of consumer-technology services, ranked the system failure as extreme and posted maps indicating large numbers of customers affected in the New York, Philadelphia and Washington, D.C., metro areas as well as San Francisco, Chicago and Denver. Reports of outages, according to the websites, spiked early Friday afternoon.

Some customers took to social media to discuss the outages, saying they were having trouble getting through the company’s phones and online chats. Comcast, on Twitter, directed customers to an internal website that was at one point down as well, eliciting a second round of customer complaints.